1. Field of the Invention
The present invention relates to a preparation process of a pigment dispersion useful as an ink-jet ink, and to an image forming process and an image forming apparatus using a liquid composition containing the pigment dispersion.
2. Description of the Related Art
In recent years, the digital printing technology has vigorously progressed. Typical examples of this digital printing technology include those called electrophotographic technology and ink jet technology, and its importance as image forming technology in offices, homes and the like has increased more and more in recent years.
Among these, the ink jet technology has a great feature of compactness and low power consumption as a direct recording method.
The formation of higher-quality images is also quickly advanced by formation of micro-nozzles or the like. An example of the ink jet technology includes a method in which an ink fed from an ink tank is evaporated and bubbled by heating it by a heater in a nozzle, thereby ejecting the ink to form an image on a recording medium.
Another example includes a method in which an ink is ejected from a nozzle by vibrating a piezoelectric element.
Water-soluble dye inks have been applied to these methods to date, but have involved problems of bleeding, feathering, poor weatherability and the like.
In order to solve these problems, it has been investigated in recent years to use pigment inks (see U.S. Pat. No. 5,085,698). In fact, ink-jet inks containing a pigment dispersion in their ink compositions also begin to spread.
However, the pigment inks are often poor in long-term storage stability and ejection stability from an ink jet head as compared with dye inks. In addition, images formed with pigment inks generally tend to be low in color developability compared with images formed with dye inks because they undergo light scattering and/or light reflection by the influence of pigment particles.
As a method for improving the color developability of pigment inks, it has been attempted to pulverize pigment particles. A pigment pulverized to 10 nm or smaller (hereinafter referred to as “fine pigment particles”) is less influenced by light scattering and has an increased specific surface area and is thus expected to achieve color developability comparable with that of dyes.
The pulverization of pigment particles is generally mechanically conducted by means of a dispersing machine such as a sand mill, roll mill or ball mill.
In these methods, the pulverization of the pigment is limited to approximately primary particles (about 100 nm), and so it takes a lot of time and cost for further pulverization. In addition, it is difficult to stably provide those having uniform quality (Japanese Patent Application Laid-Open No. H10-110111).
On the other hand, there has been proposed a method for preparing fine pigment particles by dissolving a pigment and then reprecipitating it.
Japanese Patent Application Laid-Open No. 2004-043776 discloses that an organic pigment and a water-soluble dispersant are dissolved in an aprotic solvent in the presence of an alkali, and this solution is then mixed with water to adjust fine pigment particles excellent in dispersion stability.
When copper phthalocyanine that is a typical cyan pigment is applied to this process, however, the solubility of copper phthalocyanine in the aprotic solvent in the presence of the alkali is low. The process has thus involved a problem that it is hard to make high the concentration of pigment derivative particles contained in the resulting liquid composition.
Japanese Patent Application Laid-Open No. 2001-262023 discloses a method for preparing a pigment with improved wettability and compatibility with respect to paper, cloth and the like. In this method, a copper phthalocyanine derivative obtained by introducing a hydrophilic substituent into copper phthalocyanine is dissolved in a supercritical solvent, and the substituent is partially eliminated from this derivative, thereby insolubilizing copper phthalocyanine to form particles. According to this process, copper phthalocyanine and the copper phthalocyanine derivative are said to be provided in a state mixed with each other.
However, this process has involved a problem that since it is essentially difficult to cause a dispersant to intervene in the process of forming the particles, it is extremely difficult to prepare fine pigment particles in a dispersed state.